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MG MGB Technical - Lazy Speedo

Whereas I am a curious but lazy guy, I wonder what causes the 1977 speedo to read so low. If I have a 1250 and it needs a 1000, would that explain it? How does a speedo show 65 mph, when the car is apparently going 75 mph? MoMa rebuilt the gauge, could the magnet be wrong? What causes a low read at speed? Thx to all you what knows. Cheers, Vic
vem myers

It can be a lazy speedo as your choice of title suggests - the originals were not that accurate. Alternatively, if you have an up-rated gearbox, you may have the wrong drive.
R Walker

Sticky needle bearings, or too strong a return spring resisting the pull of the revolving magnet. Could be just dirty, greasy or badly adjusted spring. They are really quite easy to clean and the needle/spring is simple to adjust.
Allan Reeling

What does the dial say? All RB MGBs originally had a 1000tpm speedo to go with the blue-label OD - non-OD would have had the same ratio speedo drive.

The speedos were calibrated to be between 0 and 10% fast to ensure you don't unwittingly exceed any limits. Your error is way outside that, if it was rebuilt and read that way immediately it was either done very badly, or something else is wrong. Any other MGB gearbox would over-read, by anything from 5 to 25%. The speedo drive gears could be incorrect, you could have a non-standard axle ratio, or undersized wheels/tyres. If you have a non-MGB gearbox its speedo drive ratio could be anything could be anything.
PaulH Solihull

How do you adjust the needle/spring? My speedo actually reads high. When it reads 65 I'm doing about 55 mph. It is the correct model for my 3 synchro non-OD box. The error is pretty much proportional to speed.

Robert McCoy

The fixed end of the spring should be on an arm that can be bent one way or the other to increase or reduce tension on the spring, if I remember correctly. If the error is proportional that should do the trick. If it's more a case of being a fixed amount out then repositioning the pointer on the spindle should sort that. I took mine out of the case but couldn't detach the odometer reset first. While fiddling it out the pointer pinged off, I thought it had snapped the spindle but fortunately it hadn't. I jacked up a rear wheel and ran it at a calculated 30mph as indicated by the revs in 4th, replaced the needle at the 30mph position, and it proved to be spot-on.
PaulH Solihull

Vic - If you have a 1250 TPM speedo in a car that calls for a 1000 TPM, the speedo will read slow. If you take it back to the shop that you had the work done at, they might be able to recalibrate it for the 1000 TPM, but it may need a different spring to do so. Cheers - Dave
David DuBois

Many thanks all you. Good lead all. I'll see to it and let you know. Cheers, Vic

vem myers

This thread was discussed between 02/01/2012 and 06/01/2012

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